## 0x01 继承AbstractProcessor抽象类

当定义好Annotation注解后，接下来就需要一个注解处理器来处理我们的自定义注解了。实现Java Annotation一般需要继承AbstractProcessor抽象类，并且重写其四个方法来实现提取，解析并处理自定义注解的逻辑如下：

class WondertwoProcessor extends AbstractProcessor {//返回注解处理器可处理的注解操作@Overridepublic Set<String> getSupportedOptions() {return super.getSupportedOptions();}//得到注解处理器可以支持的注解类型@Overridepublic Set<String> getSupportedAnnotationTypes() {return super.getSupportedAnnotationTypes();}//执行一些初始化逻辑@Overridepublic synchronized void init(ProcessingEnvironment processingEnv) {super.init(processingEnv);}//核心方法，扫描，解析并处理自定义注解，生成***.java文件@Overridepublic boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {return false;}
}

## 0x02 重写核心方法process()

由上可知process()方法才是扫描，解析，处理注解的核心方法，动手实战一下写一个简单的WondertwoProcessor来提取自定义注解@CustomizeInterface，然后借助JavaPoet生成Java接口文件。

/*** 自定义注解处理器，将类中public方法提取为接口方法（不含static方法）* {*     Exec: apt -factory annotation3.WondertwoFactory*     ProvinceDefiner.java -s ../annotaion3* }* Created by wondertwo on 2016/10/18.*/
class WondertwoProcessor extends AbstractProcessor {private ProcessingEnvironment envir;public WondertwoProcessor(ProcessingEnvironment env) {this.envir = env;}@Overridepublic Set<String> getSupportedOptions() {return super.getSupportedOptions();}@Overridepublic Set<String> getSupportedAnnotationTypes() {return super.getSupportedAnnotationTypes();}@Overridepublic synchronized void init(ProcessingEnvironment processingEnv) {super.init(processingEnv);}@Overridepublic boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {for (TypeElement typeEle : annotations) {WondertwoInterface wondertwoInterface = typeEle.getAnnotation(WondertwoInterface.class);if (wondertwoInterface == null) break;Class clazz = typeEle.getClass();if (clazz.getDeclaredMethods().length > 0) {try {if (typeEle.getModifiers().contains(Modifier.PUBLIC)&& !typeEle.getModifiers().contains(Modifier.STATIC)) {PrintWriter writer = (PrintWriter) envir.getFiler().createSourceFile(wondertwoInterface.value());writer.println("package " + clazz.getPackage().getName() + ";");writer.println("public interface " + wondertwoInterface.value() + " {");for (Method method : clazz.getDeclaredMethods()) {writer.print("    public ");writer.print(method.getReturnType() + " ");writer.print(method.getName() + " (");int i = 0;for (TypeParameterElement parameter : typeEle.getTypeParameters()) {writer.print(parameter.asType() + " " + parameter.getSimpleName());if (++i < typeEle.getTypeParameters().size())writer.print(", ");}writer.println(");");}writer.println("}");writer.close();}} catch (IOException e) {throw new RuntimeException(e);}}}return true;}
}

看过《Java编程思想》的同学肯定对上面的实例非常眼熟，书中对应的实例也是提取非静态公有方法生成接口源文件，但由于是JDK6.0标准已经有很多API发生了很大的变化，本例基于JDK8！

可以看到我们只在process()方法中加入了处理注解，生成.java文件的逻辑，这里是的逻辑是根据自定义注解提取对应类的非静态public方法，然后将抽取的非静态共有方法拼接成对应的接口！

## 0x03 实例探究：Android依赖注解库ButterKnife

不会偷懒的程序员不是一个好程序员，Android开发者对ButterKnife依赖注解库一定耳熟能详，当我们UI布局中控件很多的时候ButterKnife无疑显著提高了开发效率。

作为一个注解库其实现的原理依然是Java Annotation的方式，我们在Github翻出ButterKnife源码文件，找到其核心类——注解处理类ButterKnifeProcessor.java，源码较长删减后如下：

public final class ButterKnifeProcessor extends AbstractProcessor {@Override public synchronized void init(ProcessingEnvironment env) {super.init(env);elementUtils = env.getElementUtils();typeUtils = env.getTypeUtils();filer = env.getFiler();}@Override public Set<String> getSupportedAnnotationTypes() {Set<String> types = new LinkedHashSet<String>();types.add(Bind.class.getCanonicalName());for (Class<? extends Annotation> listener : LISTENERS) {types.add(listener.getCanonicalName());}types.add(BindBool.class.getCanonicalName());types.add(BindColor.class.getCanonicalName());types.add(BindDimen.class.getCanonicalName());types.add(BindDrawable.class.getCanonicalName());types.add(BindInt.class.getCanonicalName());types.add(BindString.class.getCanonicalName());return types;}@Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {Map<TypeElement, BindingClass> targetClassMap = findAndParseTargets(env);for (Map.Entry<TypeElement, BindingClass> entry : targetClassMap.entrySet()) {TypeElement typeElement = entry.getKey();BindingClass bindingClass = entry.getValue();try {JavaFileObject jfo = filer.createSourceFile(bindingClass.getFqcn(), typeElement);Writer writer = jfo.openWriter();writer.write(bindingClass.brewJava());writer.flush();writer.close();} catch (IOException e) {error(typeElement, "Unable to write view binder for type %s: %s", typeElement,e.getMessage());}}return true;}@Override public Set<String> getSupportedOptions() {return Collections.singleton(OPTION_SDK_INT);}
}

如果想要进一步了解ButteKnife扫描，解析，处理注解，生成Java代码的每一部细节，可以参考文章：浅析ButterKnife